1. Field of the Invention
The present invention relates to a communication apparatus which communicates through a plurality of data transmission channels.
2. Description of the Related Art
In these days, the demand for digital-radio data communication has been increasing. To respond to the demand, spread spectrum communication, which is one of digital-radio communication methods, has been used. In spread spectrum communication, slow frequency hopping has frequently been used due to its simplified configuration.
In slow frequency hopping, the used frequency is changed for each set of a plurality of data bits to reduce effects of narrow-band noise when data is re-transmitted if an error occurs, and to obtain a frequency-diversity effect. However, the sending and receiving sides are required to be synchronized to effect frequency switching.
Conventionally, to get synchronization, the receiving side slightly converts the phase of a hopping pattern until the phases of the hopping patterns used in the sending and receiving sides match. Use of a matched filter has been proposed for high-speed synchronization.
The above-described conventional system has drawbacks, complicated hardware as well as long synchronization-acquisition time.
When there are a plurality of hopping patterns and a plurality of radio communication channels, the same frequency may be used at the same time for different transmissions, increasing the data error rate.
Due to these drawbacks, frequency-hopping multiple access could not be readily implemented. As a result, it was difficult to use available frequencies efficiently, and it is hard to configure a system having large transmission capacity.